1. Field of the Invention
This invention relates to a dry etching method used for production of semiconductor devices exhibiting a high integration degree. More particularly, it relates to a method for anisotropic etching of a polycide film used in a gate electrode or the like without using a chlorofluorocarbon gas, referred to hereinafter as a CFC gas.
2. Description of Related Art
Polysilicon has hitherto been used extensively as a gate electrode material for LSIs. However, in keeping up with a demand for a high speed operation, such as a shorter access time in a memory device with a high integration degree, silicides of a refractory metal, having a resistance value about one digit lower than that of the polysilicon, have come to be used. For constructing the gate electrode layer with the use of the silicide of refractory metals, a so-called polycide film structure is predominantly employed, in which, in consideration of interfacial characteristics with the gate insulating film which are most likely to affect the characteristics of the device and operational reliability, a doped polysilicon (DOPOS) layer, exhibiting proven effects, is formed on the gate insulating film, and a layer of a polycide of refractory metals is formed on the DOPOS layer.
However, such polycide film has presented new difficulties in the art of dry etching, because it has to exhibit anisotropy with respect to both of two different materials. That is, on etching the polycide film, undercuts or constrictions tend to be produced in a pattern because a lower layer of polysilicon is etched more promptly than an upper layer of the refractory metal silicide due to the differential vapor pressure of produced halogen compounds, and also because a reaction layer is formed in an interface between the polysilicon layer and the refractory metal silicide. These shape abnormalities tend to produce offset regions obstructing introduction of impurities at the time of an ion implantation carried out for formation of a source-drain region, as well as to lower dimensional accuracy at the time of a formation of a sidewall carried out for realization of an LDD structure, and hence are not allowable particularly with submicron size devices.
Heretofore, a CFC gas, such as CFC 113 (C.sub.2 Cl.sub.3 F.sub.3) has been used most extensively as an etching gas for policide films, as reported for example in Semiconductor World, issued by Press Journal Inc., 1989, October, pages 126 to 130. This gas, containing chlorine and fluorine atoms in its molecule, allows an etching reaction to proceed efficiently in both the radical mode and the ion mode, and enables high anisotropy etching simultaneously with sidewall protection due to deposition of a carbonaceous polymer.
However, the CFC gas has been pointed out as destroying an ozone layer of the earth and its production and use are scheduled to be banned in the near future. Thus it is mandatory to find a suitable substitute of the CFC gas in the field of dry etching and to establish a method for its effective utilization, that is, a process in which the CFC gas is not used.
In view of demands for refinement of device size and for finding a substitute for CFC gas, HBr has recently attracted attention as an etching gas. For example, it has been reported in Digest of Papers 1989 Second MicroProcess Conference page 190 that a satisfactory shape anisotropy has been achieved by a reactive ion etching of an n.sup.+ type polysilicon layer with the use of HBr. Br is an etchant highly effective for achieving aniosotropy because it is capable of producing an etching reaction when aided by ion bombardment, even though it has a larger atomic radius and cannot intrude into crystal lattices or grain boundaries of a material to be etched and hence it cannot produce a spontaneous etching reaction without difficulties.
However, it has been found that dry etching with HBr presents problems when used for etching the polycide film because bromides of the refractory metals sputtered out during etching of the refractory metal silicide layer contaminate an etching chamber. Further because Br radicals are intrinsically low in reactivity when used as an etchant, the etch rate is markedly lowered as compared to that when a conventional CFC gas is used for etching.
For overcoming these problems, a group of researchers, of which the present inventor is a member, has made a report on a dry etching method for a polycide film by an etching gas including HBr admixed with fluorine-containing gases, such as SF.sub.6 (Extended Abstracts, second volume, page 460 , title number 28p-ZF-3, in 37th Spring Meeting (1990) of the Japan Society of Applied Physics and Related Societies; and extended papers, second volume, page 463, title number 26p-ZF-3, in the 51th Autumn Meeting (1990), of the Japan Society of Applied Physics). It is envisaged to effect etching with this etching gas at a reasonable speed by supply of F* as well as to achieve high anisotropy by the effect of sidewall protection by a reaction product CBr.sub.x. In the above report, reference has been made to overetching with only HBr after the etching of the polycide film is well-nigh terminated. It is envisaged to eliminate the polycide film from the wafer in its entirety without markedly lowering the overall etch rate while maintaining a larger selectivity with the gate oxide film.
Meanwhile, in the field of dry etching, a so-called low temperature etching, in which a substrate to be etched is maintained at 0.degree. C. or lower during etching, has recently attracted attention. It is intended with the low temperature etching to prevent shape defects, such as side etching, by terminating or restraining a radical reaction at a sidewall section, with an etch rate along the depth being maintained by an ion assist effects. If the technique is used for etching of the polycide film by SF.sub.6 /HBr gas system, the reaction product SiBr.sub.x may be deposited besides the reaction product of the resist material with Br and may be utilized as a sidewall protection film.
However, subsequent researches have revealed that problems are left to be solved in the practical utilization of dry etching making use of a mixed gas of a fluorine gas typified by SF.sub.6 /HBr and HBr.
In the first place, etching at room temperature presents problems in that high anisotropy cannot be attained, unless the proportion of HBr in the etching gas is high to some extent, so that the etch rate is essentially low. On the other hand, if the proportion of HBr is raised to make much of anisotropy and accordingly a large quantity of the reaction product SiBr.sub.x is produced, the latter tends to be deposited on the sidewall etc. of the resist pattern. The reaction product which is left after removal of the resist pattern by ashing tends to be dislocated to cause particle pollution. Since HBr itself is a dangerous and highly hygroscopic compound, process stability may be affected if HBr is used in larger quantities.
In the second place, undercuts may be produced in the polysilicon layer during overetching.
This phenomenon is explained by taking an example of a process for formation of a polycide gate electrode shown in FIGS. 2A and 2B. Referring to FIG. 2A, an doped polysilicon layer 13 and a refractory metal silicide layer 14 containing a refractory metal, such as tungsten, are sequentially deposited on a semiconductor substrate 11 of silicon etc. with an interposition of a gate insulating film 12, and the so-formed substrate is subjected to dry etching, using SF.sub.6 /HBr as an etching gas and using a prepatterned photoresist pattern 15 as a mask, until the polysilicon layer 13 is etched by about one-half its depth. At this stage, a sidewall protection film 16 is deposited on sidewalls of the refractory metal silicide layer 14 and the polysilicon layer 13 for maintaining shape anisotropy. The sidewall protection film 16 is made up e.g. of reaction products between Br and the resist material of the photoresist pattern 15, or SiBr.sub.x which is a reaction product between Br and Si which is mainly supplied from the polysilicon layer 13.
However, when etching is well-nigh terminated and overetching is started, the polysilicon layer 13 as a Si source is decreased, so that the deposited amount of SiBr.sub.x is decreased. Moreover, Br* which is in excess relatively due to loss of counterparts of bonding tends to attack the sidewall section of the polysilicon layer 13. This produces an undercut 17, as shown in FIG. 2B, so that the pattern width of the polysilicon layer 13 becomes narrower than the desired pattern width. Under certain situations, these undercuts may be produced by the raised etch rates at the time point when etching of the refractory metal silicide layer 14 is terminated and the surface of the polysilicon layer 13 is exposed. Such phenomenon cannot be suppressed effectively by application of low temperature etching.
Switching the etching gas to a composition consisting solely of HBr for overetching for preventing the undercuts has been proposed by the group of researchers, of which the present inventor is a member. Similar effects may also be achieved by switching the gas composition on exposure of the polysilicon layer. However, since etchants are present in excess amount during overetching, it is necessary to provide more intensive sidewall protection than in the course of intrinsic etching. The above mentioned switching technique is not fully satisfactory in this respect. In addition, difficulties are raised as to end-point detection in switching the gas composition at an interface between the refractory metal silicide layer and the polysilicon layer. It is because no useful emission species of refractory metal halides have been known in the measurement of an emission spectrum which is usually employed for monitoring dry etching.